Britbike forum Forums British motorcycles in general British motorcycles in general '72 Fork Dyno Test

Put together an early OIF drum brake slider and stanchion with the second damper tube, the one with single slot at the bottom. The first damper tube had two slots further up near the damper head.
The early damper tube would have quite a bit of difference due to the slots causing the oil to bypass the damper head by the middle of the stroke.
The dyno stroke is 2" so I made runs near the top (TOS) and bottom (BOS) of the fork's travel to see any differences. On these plots TDC is top of compression and BDC is extended. The curves should be read as a clockwise movement around the trace.
First it was run with no oil but with a film of WD-40 to wet the O-ring and seal.

Run 1 is TOS and run 2 is BOS. There is a difference is stiction between the two.
Next I filled the fork with WD-40 which would represent a very light oil: Both are BOS.

Next was Ford ATF. Here is Run 2, 3 and 4. All BOS:

Seeing more damping with compression. The stiction magnitude is as much as the rebound damping.
Looking at the ATF damping at TOS and BOS gives this:

Run 5 at TOS shows virtually no damping except for the stiction.
Changing to fork oil shows slightly less damping than ATF. At TOS:

and BOS:

This was oil taken from my T595. I have to check what weight was used.
The fork oil was drained and a run to compare to the first run, TOS:

BOS:

Notice the reduced stiction on the rebound of the stroke compared to the top. Possibly more oil has got to the seal but that is not seen at the TOS runs.
Many things to try. I have teflon rings to replace the damper head O-ring, a new damper rod/head with stacked reed valves and my three piece slider with hard anodized tube.

It would be nice if I knew what I was looking at. I used F type ATF for a long time, only recently switched to dedicated fork oil, 10 weight in the Bonneville, 15 in the Trident.

One of the recommended oils was Castrol QTF (still available) I don’t know what the differences in viscosity are between that and other ATFs, I doubt there would be much between them but used the QTF in my 71 A65.

I think progressive springs single-handedly make a huge difference, as does oil quantity, and often the measurement on the side of a cup (or even lab beakers) are no where near what they should be. Measurement by weight would be better.

Time to go to shock dyno school and learn to understand what we're looking at.

The machine starts collecting data at the bottom of the stroke.The yoke compresses the shock then returns to extension.
Looking at the fourth plot (Runs 4-5) with ATF, cycling the fork near the top of the stroke the big jump in force from the bottom of the stroke is from the stiction of the seal and damper head O-ring, probably some additional friction between the stanchion and slider although there is no lateral force applied to the fork as in the real world. The force is relatively flat up to the top of the stroke which is drag from the seals. When the stroke reverses at the top of the stroke the drop in the curve is from reversing the stiction force. Again, not much change in force during the return stroke.
Run 4 near the bottom of the fork travel shows more force change with stroke. The applied velocity is sinusoidal with the maximum velocity of the stroke at zero on the plots. The curve is almost linear so I suspect the change in force is more due to the change in air pressure than damping. I should have included force-velocity plots which would make this more apparent. The return part of the stroke has more curvature showing some damping effect.
I have to check why the curves are displaced from zero at the start of the stroke. Normally the machine is not running at the start of a single run. Because I am working on an interference problem with the motor control shutting down the databox USB the data is collected with the machine running at startup. However, when taking a series of runs the machine is constantly running so that should not be the reason.
I believe Steve has collected data from a modern RSU fork. I will see if I can include that for comparison.

This is very interesting. I have toyed with the idea finding a modern fork seal that fits the fork tube diameter and making an adapter to fit it to the slider. Your machine would characterize the improvement.

This is very interesting, Dave. I’m so glad you’ve undertaken to graph the fork behavior to render it into a visual format. It’d be very useful to compare any of these iterations of the 50 year old Triumph design with any modern upside down cartridge sport bike fork, to show what something much closer to state of the art looks like, and how it differs from what our older forks provide. As you said above, stiction, for instance, is a major factor in total damping on the Triumph fork and the result is an odd damping profile, about even in compression and rebound, with more friction induced “damping” on compression than might be optimal. For instance, I’m trying to figure out if creating or adapting a cartridge damping system to the T140 fork even makes sense, since the ever present high stiction in the Triumph fork would be so much of a factor and defeat efforts to make the damping more modern in function.

What I’ve come to using all stock 1979 Triumph parts before trying any mods is simply, 5 weight oil in nearly stock amounts (225 cc) to avoid compounding the effect of stiction on compression through too thick oil or too much air spring effect, just regular old fork seals (not “leak proof) and progressive springs, to solve the “way too soft spring” problem, to attempt to limit too much stiction and too soft compression. Rebound? As a practical matter, a lot of stiction seems to supplement the 5 weight oil which in normal circumstances would be too light. In other words it not much but it’s what I got to. As is obvious, there’s lots of better forks out there, but these are what Triumph gave us.

Keep it up David, lots of food for thought here.

Dave, did you compare stiction caused by the stock O rings with that caused by those wavy washers you sent to me years ago? I put those in the disc forks I had on my A65 at the time, now on my T150V.
I had Progressive springs in those forks when they were on the BSA. They need some spacers to work on the heavier Trident, so I went with stock springs. I don't really ride fast, I'm not sure that I could tell much difference, but those springs seemed to top out a lot on the Trident.

F type ATF has become rare and expensive in the US. That's the big reason I now use Belray fork oil. 190cc in the Bonneville, 220cc in the Trident.

I checked with Steve, the offset of the Bottom of Stroke (BOS) runs is from the air pressure in the shock. If I did not have the slider all the way extended when putting the cap on or a slight difference in where in the stroke of the fork the machine tested can account for differences between the ATF and fork oil runs initial force.
I have the data for the ATF and fork oil runs plotted in force versus velocity. there are two ways to show this, as absolute velocity or signed velocity. Here are runs 4-5 ATF in signed:

and absolute:

Runs 7-8 fork oil:


Looking at signed velocity, the force starts at zero (for TOS, offset by air pressure on BOS). Move along the curve to the right, reaching maximum velocity, then back to zero velocity, to maximum negative velocity and back to zero.
Looking at absolute velocity, the negative velocity part of the curve has been folded over to the positive side.
Notice these curves look nothing like the rear damper where there is a change in sign of the damping with the velocity direction.
The seal in this slider is very old. Changing to a new seal might reduce the stiction. TCN or DLC coating on the stanchion might also help.

DP - I will check the effect of the teflon damper head seals.

Removing the rubber seal protector caps from late forks and replacing with gaiters made a big improvement for me stiction-wise

This slider has only the top seal and it is not the floating type.

I mess about with dirt bikes, so spend a bit of time chatting to my suspension man. Getting the stiction down is the first step to any upgrade . He advises boring out the slider to take brass loaded Teflon bushes and adapting a modern fork seal to the slider. Beyond that the sky's the limit to how far you want to go.

He has put modern internals on T140 forks.

Very interesting Dave. Id be interested in seeing the Cannondale Lefty mtb “strut”. It uses needle rollers instead of bushes that run on a square stanchion. As a mountain bike rider I had both conventional and lefty and found the lefty to be superior on bumpy tracks as there was very little binding with side loads eg. Any bumpy corners.

SA- that is the reason that I made these:

The top cap has a bronze bushing and the bushing shown on the right side threads into the end of the stanchion. The tube in the middle is hard anodized. This allows the stanchion to flex between the two. The stock slider forces the stanchion to straighten in the very top of the slider which causes a lot of friction.
This is what DP was asking about:

The teflon rings are typically used as O-ring back-up rings.The damper head on the right has reed valves to control the compression and rebound. Selecting the stack of reeds allows setting each effect. The screw in the top allows external adjustment.
I think the first MotoCzysz motorcycle had sliders running on roller bearings with a rear shock handling the damping.

Indeed. My man also states the bushes should be as short as you dare, for the reasons you have stated.

As well as a former road racer, I’m a dirt bike guy too. If there were something for the Triumph that looked stock from the outside but contained parts like my modern Yamaha YZ mx forks, meaning cartridges, Teflon bushings, and modern seals, well… or like my Yamaha R-1 road racer, same parts…with that we’d have it solved. That’s basically what Dave has made, really nice looking stuff. I’m interested in how that goes, Mr. Madigan.

Peter at Promecha in Melbourne, Australia has done a full modern internals conversion for classic bikes. He is happy to chat on the phone.

Made another set of runs.All runs start at 415mm from top of stanchion to top of the fork seal and used the Showa SS8 fork oil which is equivalent to 10W. The oil is about 200cc and measured 60mm above the damper head. when replacing parts the oil was filled to the same height below the top of the stanchion to account for any oil left in the parts. Each plot has 12 runs to check for consistency in the data.
Stock damper with O-ring seal on damper head 6 in/sec


10 in/sec


Stock damper with teflon damper seals 6 in/sec


10 in/sec


The jump at the ends of the force-stroke plots appears to be less with the teflon seals. The damper rod with the teflon seals will drop through the stanchion under its own weight. The O-ring will not.

Changed to the damper head that I made. Same conditions.6IPS


With a change in the shims:


This looks like a better arrangement of shims. The plots look more like the rear shock. It will take a lot of experimentation to get a suitable set of shims. The damper head may need to be redesigned. There are three transfer ports for each direction. Bending flat shims at three points takes more force than two. This could be the cause of the wiggles near zero velocity, neglecting the stiction jump. Shims are available in different diameters and thickness.
I also tested the effect of the damper adjustment screw. 6IPS:


This is not a good result. Holes do not have a non-linear effect which is why most modern dampers use shims.
At 10 IPS the effect is worse


A possible problem with the late damper top nut is the hole out the top. The first damper nuts had the hole out the side. When the forks are being worked hard the oil is being sprayed up at the cap nut which can cause the damper (such as it is) to draw air instead of oil.